1. Field of the Invention
The present invention relates to a pressurized-fluid-operated, piston-cylinder-type, linear actuator having a plurality of piston surfaces within a unitary cylinder for providing a higher output force for a given cylinder diameter and a given pressure of the pressurized fluid. More particularly, the present invention relates to a pressurized-fluid-actuated actuator of compact size and that includes a single movable piston having two axially spaced pressure surfaces for increased actuating force and for movement of the piston in a first direction, and an additional, single pressure surface for moving the piston in a second direction opposite from the first direction.
2. Description of the Related Art
Pressurized-fluid-operated linear actuators incorporating pistons movable within cylinders are well known in the art and are used for many different purposes, including providing sufficient force to actuate a device or to move one or more members of a combination of elements. Generally, the output force provided by such actuators can be increased either by increasing the pressure of the fluid utilized to operate the actuator, or by increasing the surface area of the piston, which also requires an increase in the diameter of the cylinder within which the piston is contained. At times, however, the available space within which an actuator must be positioned is very limited, and often the available space is not sufficient to allow an increase in the cylinder diameter. Additionally, at other times the available pressure of the pressurized fluid may be inadequate to provide the desired output force from an actuator having a given diameter. It is therefore desirable to be able to provide an actuator structure that will enable a small actuator to provide the output force of a larger diameter actuator, or to be able to provide the same or a higher output force when supplied with pressurized fluid at a lower pressure.
A number of actuator structures have been devised to respond to the problems noted above. For example, in U.S. Pat. No. 3,880,051, entitled xe2x80x9cPneumatic System Including Auxiliary Output,xe2x80x9d which issued on Apr. 29, 1975, to Eppler, there is disclosed a dual chamber cylinder in each chamber of which a separate, independent piston is slidably provided. The piston rod of one piston extends into the adjacent chamber and includes an axial passageway through the piston and piston rod to allow the fluid pressure at the head end of one piston to be communicated to the head end of the second piston while the piston rod of the first piston is in contact with the head of the second piston. As a result, the output force of the piston rod of the second piston is multiplied without an increase in either the cylinder diameter or the pressure of the operating fluid.
Another form of multiple chamber linear actuator is disclosed in U.S. Pat. No. 3,752,040, entitled xe2x80x9cMulti Piston Power Pack Unit for Fluid Actuated Tool,xe2x80x9d which issued on Aug. 14, 1973, to Pawloski et al. This reference shows a force-multiplying actuator structure in which two axially spaced pistons that are physically interconnected are slidably carried in respective chambers within a single cylinder. The cylinder of the actuator is divided into two chambers by a fixed, interior dividing wall, and pressurized fluid from the head end of one chamber is communicated to the head end of the adjacent chamber by an axially-extending passageway that passes through the connecting member that interconnects the respective pistons.
Additional types of multiple chamber linear actuators are disclosed in U.S. Pat. No. 5,191,825, entitled xe2x80x9cLow-Impact Air Cylinders,xe2x80x9d which issued on Mar. 9, 1993, to Beneteau et al., and in U.S. Pat. No. 5,483,796, entitled xe2x80x9cFluid Cylinder,xe2x80x9d which issued on Jan. 16, 1996, to Ando. In each of those patents three coaxial pistons are provided within a single outer cylinder for providing increased output force. In the Beneteau et al. patent two of the pistons are interconnected, and each piston is carried in a separate chamber. One of the two interconnected pistons is slidably received within the third piston. In the structure disclosed in the Ando patent the three pistons are concentrically disposed within a cylinder that does not include a fixed inner dividing wall.
A further form of multiple chamber linear actuator is shown in U.S. Pat. No. 5,368,470, entitled xe2x80x9cMultiple Pin Closure Nozzle Assembly for Injection Molds,xe2x80x9d which issued on Nov. 29, 1994, to Mxc3xa4nner and U.S. Pat. No. 5,375,994, entitled xe2x80x9cPiston Driven Pin Closure Nozzle Assembly,xe2x80x9d which issued on Dec. 27, 1994, to Friderich et al., as well as Japanese Patent Publication No. 4-320820, entitled xe2x80x9cMold Device for Injection Molding,xe2x80x9d which was published on Nov. 11, 1992. In these references, the pressurized fluid cylinder is divided into two pressure chambers (three chambers in the Friderich patent), within each of which is disposed a respective drive piston for multiplying the output force of the actuator by combining the output forces provided by the individual pistons.
Although the art discloses various structures for providing increased output force from a pressurized-fluid-operated actuator, the structures shown in each of the above-identified references require either a significant increase in the axial length of the actuator, or a significant increase in the diameter of the actuator cylinder, or they involve a complex structure having many internal parts. As a result, the disclosed structures have limited applicability in confined spaces.
It is therefore an object of the present invention to overcome the shortcomings of the prior art devices and to provide a pressurized-fluid-operated cylinder that can provide a desired increased force output without significant enlargement of the size of the actuator cylinder.
It is another object of the present invention to provide a compact linear actuator for operating a valve pin in a valve gate of an injection mold assembly.
Briefly stated, in accordance with one aspect of the present invention an actuator is provided that includes an outer cylinder of tubular form that defines an inner cylindrical surface and that has a longitudinal axis. A piston is slidably carried within the outer cylinder for axial movement therewithin and has a rod affixed thereto, the rod extending from the piston in an axial direction relative to the outer cylinder. The piston divides the outer cylinder into a head end chamber and a rod end chamber that is axially spaced from the head end chamber. The piston also includes an inner cylindrical space.
A dividing wall extends transversely across the inner cylindrical space within the piston and at a fixed axial position relative to the outer cylinder, thereby dividing the inner cylindrical space into a first inner chamber and a second inner chamber. A first fluid conduit is in fluid communication with the head end chamber of the outer cylinder and with the first inner chamber for moving the piston and rod in a first axial direction relative to the outer cylinder, to cause the rod to move in a outward direction relative to the outer cylinder to provide a rod extension stroke when pressurized fluid is introduced into the first fluid conduit.
A second fluid conduit is in fluid communication with the second inner chamber for moving the piston and rod in a second axial direction relative to the outer cylinder and opposite from the first axial direction, to cause the rod to move in an inward direction relative to the outer cylinder and to provide a rod retraction stroke when pressurized fluid is introduced into the second fluid conduit.